Histopathology of ARDS is defined as diffuse alveolar damage (DAD). Antemortem evidence for fibroproliferation directly correlates with risk of death in ARDS, and postmortem findings in the lungs of patients who died of ARDS demonstrate increased fibroproliferation. Fibroblasts are the major cellular source of extracellular matrix. However, the origin of the interstitial fibroblast that contributes to fibrosing alveolitis during DAD remains to be elucidated. Our preliminary data demonstrates that circulating fibrocytes play a significant role in promoting the fibrosing alveolitis of Acute lung injury (ALI). We hypothesize that circulating fibrocytes, mobilized from a bone marrow (BM) precursor cell, extravasate into the lung by a chemokine-dependent (CXCL12/CXCR4) mechanism, differentiate to tissue myofibroblasts, and contribute to fibroproliferative response of ALI. To test this hypothesis, we will perform experiments in the following Specific Aims I. A) To determine the origin of circulating fibrocytes and whether they are derived from a BM progenitor cell. B) To demonstrate that the temporal mobilization of BM progenitor fibrocytes, extravasation of circulating fibrocytes, and differentiation into myofibroblasts directly correlates with the fibroproliferative phase of ALI. II. A) To determine whether factor(s) generated in the lung promote mobilization of BM precursor fibrocytes and increase the pool of circulating fibrocytes during ALI. B) To establish that the temporal expression of CXCL12 directly correlates with the kinetics of extravasating fibrocytes in the lung during ALI. C) To ascertain whether homing and extravasation of circulating fibrocytes into the lung during ALI is dependent on CXCL12/CXCR4. D) To establish whether combined inhibition of BM precursor fibrocyte mobilization and recruitment of circulating fibrocytes leads to additive or synergistic attenuation of the fibroproliferative response during ALI. III. A) To establish whether hypoxia-inducible factor-1 alpha (HIF-1alpha) regulates the expression of CXCR4 on BM progenitor fibrocytes and circulating fibrocytes. B) To determine whether fibrocytes derived from BM precursor cells genetically HIF-1alpha null or von Hippel-Lindau tumor suppressor protein (VHL) null correlate with altered CXCR4 expression and recruitment to the lung during ALI. IV. A) To ascertain whether chemokine signaling through phosphoinositide 3-kinase (PI3K) is critical to fibrocyte migration, proliferation, and survival in vitro. B) To determine whether fibrocytes derived from BM precursor cells genetically devoid of PI3K (PI3Kgamma-/-) have impaired homing, extravasation, proliferation, and survival in the lung during ALI. The findings of these studies may lead to novel intervention to attenuate the fibroproliferation and reduce morbidity and mortality related to ARDS.